This invention relates to print wire actuating devices for the print head assembly of dot matrix type printers.
Generally, in the print head assemblyof assembly of type of printer, plunger type solenoids are employed, one for each of print wires, as means for selectively impacting one or more of print wires against a paper document or other recording medium. As is well known, with such print wire actuating devices, spring means are indispensable to serve the purpose of restoring the plunger each time the solenoid is deenergized and, in practice, use has been made of different forms of restoring spring, including springs of coiled form employed with printers of relatively low speed of operation and disc type springs employed with highspeed printers and including a central portion fixedly secured to the plunger and a cross or star-like formation of spoke portions extending radially outwardly from the central portion.
Use of a coiled spring in a print wire actuating solenoid as plunger-restoring means has been disadvantageous in that it necessitates a more or less increase in dimensions of the plunger and particularly in axial length thereof and results in a considerable increase in bulk and weight of the plunger unit, rendering the device practically impossible to operate at any high speed desired.
On the other hand, disc type springs of conventional design employed for high speed printing operation have involved various disadvantages, as described below. First, the configuration and the fixed, mounting of this type of restoring spring set a certain limit to the stress allowable in the spring material, precluding any increase in length of stroke of the plunger. Further, the fixed mounting of the spring on the plunger not only makes the fabrication process complicated, causing increase in initial cost, but also gives only a limited reliability, involving the danger of the spring being dislodged during operation.
In addition, with conventional forms of plunger type solenoids employed as print wire actuating means, any design aimed at reducing the size and weight of the solenoid device, which includes a plunger, normally cylindrical in shape, has caused reduction in plunger face area and hence in force of attraction, making it difficult to obtain a driving energy large enough for the plunger to operate with a high responsivity, and thus reduction in size and weight of the solenoid device has been definitely limited. Moreover, with previous forms of solenoid assembly, the plunger has usually been guided for axial movement by means of the coil spool and thus subjected to a substantial frictional drag. Owing to this, the plunger has been liable to be worn down rather rapidly in high speed operation and it has been difficult to maintain the stability in operation of the device or to lengthen the service life thereof.